1. Field of Invention
The present invention relates to decreasing EMI noise that electronic apparatuses generate.
2. Description of Related Art
Noise generated by various types of electronic apparatuses may cause undesirable interferences (EMI(Electromagnetic Interference)) against other electronic apparatuses. Various types of regulations are therefore stipulated for the noise (hereinafter “EMI noise”) generated by the electronic apparatuses.
When related art electronic apparatuses are developed and manufactured, manufacturers may intend to decrease the EMI noise generated by the electronic apparatuses so that the electronic apparatuses to be manufactured meet EMI noise regulations.
The electronic apparatus generally operates in accordance with one type or a plurality of types of clocks used as a reference. FIG. 3 is a schematic showing an exemplary frequency spectrum of a clock. As indicated with the solid line, the frequency spectrum of the clock usually has the peaks of the amplitudes at the oscillation frequency (fundamental wave: f1) of the clock and the frequencies (f2, f3, . . . ) corresponding to each of its harmonics. This causes the frequency spectrum of the EMI noise occurring in the electronic apparatus as well to generally have the peaks of the amplitudes at the oscillation frequency of the clock (f1) and the frequency (f2, f3, . . . ) corresponding to each of its harmonics. In order to decrease the EMI noise, the peaks of the amplitudes occurring at each of the frequencies of the fundamental wave and its harmonics are required to be decreased.
One way to realize this is to vary the oscillation frequency of the clock, as indicated with the dashed line in FIG. 3. The frequency spectrum is spread to decrease the peaks of the amplitudes of the frequency spectrum generated at the frequencies of the fundamental wave (f1) and its harmonics (f2, f3 . . . ) of the clock. Hereinafter, varying the oscillation frequency of the clock to spread the frequency spectrum is referred to as “spread spectrum”. The amount of variation in the oscillation frequency of the clock is referred to as “spread amount”. Generally, the more the spread amount becomes, the more EMI noise can be reduced or suppressed.